The preamp is bypassed on the X-series with the switch There was a low pass issue with electronic interference from the internal preamp even switched off on the USB model prior. So no need to remove it. Its basically just removing the warranty of the table and a bit of mass density which would hardly help overall. Same with the metal plate. Adds mass density for a more stable playback.
That's a ground loop issue. If you plug in all components involved to the same powerstrip/outlet/ground, that should go away. There's a resistive difference between the two ground points on the computer and the TT.
The hum is from a ground loop or you have poor quality cables from the turntable to the external preamp. It can be made inaudible (but still measureable in Audacity, much lower than surface noise though) without removing the preamp on this turntable, even with my kinda shitty homemade preamp (good sound but bad ground design as well). It's the older one (AT-LP120 without "X") that had more issues with preamp adding noise and that's why it looks different and that one had people removing it.
Your computer has a ton of electronic components that can produce noise for analog audio input devices via the RCA connection you are making. You can try another USB port in the back of the computer for your USB RCA crap dongle, but really you have to use the digital USB connection to your computer to receive the signal digitally without the hum/noise. And the reason you had not done that is because you can’t figure it out. There are plenty of tutorials here on RU-vid to do that with this very turntable. Also I have the same turntable and found that the built in preamp isn’t great. The Line ON switch in the back (which you need to be ON for USB connection as well) uses the not so good built in preamp of the the turntable. The AT cartridge is of the same quality, not great. Here’s how I record my vinyls and bypass all the cheap AT components that sold us this very same turntable: 1 - Got a great quality external phono preamp - Art DJ pre II for $65 (switch to PHONO in the back of the turntable) 2 - Connect a Zoom H1 Audio recorder straight to the RCAs of the new phono preamp, record in 24 bit / 48Khz and transfer to computer. 3 - (optional) but I got a new cartridge- Goldring E3 and literally doubled the sound quality, paid $150 though 😅 Good luck! Also, Dell is not great either, hence the old ASUS PC is working for you 😂
There was a reason why I didn’t use the USB out of the turntable but can’t remember why right now..but there was an issue, I’ll have to try that again.
I don’t remember why , there was a reason why but haven’t gone back and tried it again. I’ll have to update you later when reconnect to USB and see why I stopped using it,
The USB does limit the ADC to 16bit 48khz from the internal preamp. Upgrading to even a cheaper external preamp and going out analog will boast much better audio and non limiting on your ADC to capture the actual waveform and higher sample rates for extended frequency response. Playback at 16bit is fine but you want highest quality at record (HD losless). Especially if you plan on processing or edits. HD recordings sound and is closest to the source material (records) which is something to think about. The right ADC and playback gear, HD 24bit files sounds extraordinary. If in the lp groove it will be in there at those rates.
